As a rule, it is only possible to maintain the limiting values for concentrations of toxic substances in smoke gas that is released into the atmosphere, which are laid down in the regulations governing the operation of major furnace systems by treating all of the smoke gas that results from combustion in a wet-washing system, with the addition of suitable absorbents. The purified smoke gas leaves the smoke gas sulfur removal plant at a temperature that is between 40.degree. and 60.degree. C. and for this reason must be reheated to approximately 80.degree. to 120.degree. C. before being able to rise when introduced into a chimney. It is obvious that a great deal of energy is needed in order to do this. As an example, in a 70 MW power station operating under full load in excess of 2.5 million cubic meters of smoke gas has to be reheated every hour.
In order to avoid having to reheat the smoke gas that is produced in power stations, it has already been proposed that the purified smoke gases be introduced into the flow of cooling air above the heat exchange zone and the cooling air that is flowing upwards be used as a means of transporting the purified smoke gases so as to introduce them into the atmosphere. By so doing, it is possible to avoid the costs involved in reheating the smoke gases. Furthermore, it becomes possible to dispense with the construction of a chimney in power stations that are to be built. A further advantage of this procedure is the additional dilution of the smoke gases and thus the concentration of residual toxic materials that is obtained thereby. However, a prerequisite for this is a thorough and even mixing of the smoke gases that are introduced with the rising cooling air.
In a known plant of this type the introduction of the smoke gases into the flow of cooling air in the cooling tower takes place through chimney-like gas outlet pipes arranged above the heat-exchange zone in the vicinity of the center of the cooling tower; these gas outlet pipes are connected to a central smoke gas feed. The ends of the gas outlet pipes are angled so that the flow of smoke gas has a radial flow component that is directed into the cooling tower cross-section. This is indispensable for thorough mixing with the cooling air. In order to improve the smoke-gas distribution in the flow of cooling air, immediately behind the outlet cross-section of the gas outlet pipes there are insert plates; these essentially extend these pipes on the flow surface, broaden out towards their ends, and are wound spirally, being intended to swirl the flow of smoke gas that is introduced and thereby contribute to an intensive mixing with the cooling air that is moving upwards. These insert plates, which are at their end areas almost normal to the direction of flow of the cooling air, have an undesirable effect on the cooling-air flow itself. At least a part of the cooling air that flows upwards is deflected and additionally swirled. The loss of energy for the cooling-air flow that is connected with this leads, ultimately, to a reduced exit velocity and, as a direct consequence, to a smaller lift height and poorer dispersal of the mixture that emerges from the head of the tower. This can be a disadvantage, especially in the case of a changeable weather situation.
Furthermore, since the flow of smoke gases is directed relatively far in the radial sense by the inserts, the result can be that the undiluted smoke gas impacts on the walls of the cooling tower and there, because of the concentration of residual toxic substances, forms acid droplets. In order to avoid corrosion damage on the walls of the tower it is therefore essential to have a costly protective coating at these places.